Generally, a liquid crystal display has a structure in which a space between glass plates forming transparent electrodes is filled with a liquid crystal and polarizers are arranged before and after the glass plates. A polarizer used in such a liquid crystal display is manufactured in a procedure in which iodine or a dichloic dye is subjected to be adsorbed to a polyvinyl alcohol film and the film is stretched in a given direction. The polarizer thus manufactured itself absorbs light vibrating in one direction and transmits only light vibrating in the other direction therethrough to thereby produce linearly polarizing light. Therefore, an efficiency of the polarizer could not exceed 50% theoretically, which works as the greatest factor to reduce an efficiency of a liquid crystal display. As the matters worse about the absorbed light, if a liquid crystal display is operated with an increased output of a light source beyond a level, it results in inconveniences that a polarizer is broken down by heat generation due to thermal conversion of absorbed light or that a display quality is degraded under thermal influence onto liquid crystal layer in a cell.
A cholesteric liquid crystal having a circularly polarized light separating function has a selective reflection characteristic reflecting only circularly polarized light having a direction thereof coinciding with a helical rotation direction of the liquid crystal and a wavelength equal to a helical pitch length of the liquid crystal. With this selective reflection characteristic used, only a specific circularly polarizing light of natural light in a given wavelength band is transmission-separated and the other light components are reflected and recycled, thereby enabling a polarizing film with a high efficiency to be manufactured. In the context, transmitted circularly polarized light passes through a λ/4 plate and thereby converted to linearly polarizing light, and coincidence of a direction of the linearly polarized light with a transmission direction of an absorption polarizer used in a liquid crystal display enables a liquid crystal display with a high transmittance to be realized. That is, in a case where a cholesteric liquid crystal film is combined with a λ/4 plate and the combination is used as a linearly polarizer, the linearly polarizer could achieve a brightness twice as that of a conventional absorption polarizer singly used, which absorbs 50% of incident light, due to no light loss theoretically.
There has been, however, difficulty in covering all the range of visible light, since a selective reflection characteristic of a cholesteric liquid crystal is restricted to only a specific wavelength band. A selective reflection wavelength bandwidth Δλ is expressed by following formula:Δλ=2λ•(ne−no)/(ne+no)where no: ordinary light refractive index of a cholesteric liquid crystal molecule, ne: extraordinary light refractive index of the cholesteric liquid crystal molecule, and λ: central wavelength in selective reflection.
The selective reflection wavelength bandwidth Δλ depends on a molecular structure of the cholesteric liquid crystal itself. According to the above formula, if (ne−no) is larger, a selective reflection wavelength bandwidth Δλ can be broader, while (ne−no) is usually 0.3 or less. With this value being larger, other functions as a liquid crystal (such as alignment characteristic, a liquid crystal temperature or the like) becomes insufficient, causing its practical use to be difficult. Therefore, a selective reflection wavelength bandwidth Δλ has been actually about 150 nm at highest. A cholesteric liquid crystal available in practical aspect has had a selective reflection wavelength bandwidth Δλ only in the range of about 30 to 100 nm in many cases.
A selective reflection central wavelength λ is given by the following formula:λ=(ne+no)P/2where P: helical pitch length required for one helical turn of cholesteric liquid crystal.
With a given pitch length, a selective reflection central wavelength λ depends on an average refractive index and a pitch length of a liquid crystal molecule.
Therefore, in order to cover all the range of visible light, there have been adopted methods, in one of which plural layers having respective different selective reflection central wavelengths are laminated, and in another of which a pitch length is continuously changed in the thickness direction to thereby form a positional distribution of selective reflection central wavelengths.
For example, there can be exemplified a method in which a pitch length is continuously changed in the thickness direction (for example, see a publication of JP-A No. 6-281814, a specification of JP No. 3272668 and a publication of JP-A No. 11-248943). This method is such that when a cholesteric liquid crystal composition is ultraviolet exposure-cured, exposure intensities on sides of exposure and light emission are differentiated therebetween to alter a polymerization speed therebetween, which provides a change in compositional ratio of a liquid crystal composition having a different reaction speed in the thickness direction.
The bottom line of this method lies in that exposure intensities on sides of exposure and light emission are greatly different therebetween. Therefore, in many of the examples of the prior art described above, there has been adopted a method in which an ultraviolet absorbent is mixed into a liquid crystal composition so as to cause absorption thereof in the thickness direction to thereby amplify a difference in exposure dosage according to an optical path length.
A cholesteric liquid crystal film obtained by supplementary examining the prior art described above, however, showed a phenomenon that during a durability test (a heating test or a humidification test), an ultraviolet absorbent is precipitated on the surface of the cholesteric liquid crystal film or on the laminating interface to another layer. It is estimated that the ultraviolet absorbent move in the film and cohered in a long-term durability test because of the low molecular thereof. For use in general industrial materials, such the precipitation on the surface is not recognized as a wrong appearance, or even upon precipitation on the interface is not so problematic as to cause interfacial release. However, the cholesteric liquid crystal film used in a liquid crystal display is positioned in a light path of strong transmitted light, so that when such precipitations are generated, the precipitated particles are not only directly visualized, but also cause optical problems such as a reduction in the efficiency of utilization of light due to cancellation of polarized light by the precipitations, a change in light scatter distribution of a light source due to haze generated by the precipitations.
Insofar as the cholesteric liquid crystal film is used in an ordinary temperature atmosphere, generation of these precipitations is originally hardly brought about. However, when the cholesteric liquid crystal film is integrated and used in a liquid crystal display, a radiation heat from a light source in backlight is so strong that the precipitations of the ultraviolet absorbent is inevitable upon exposure to the heat for a long time. Such the precipitations when precipitated uniformly on the surface are hardly visible and hardly recognizable as defect, but the radiation heat from the light source varies highly on the surface of the liquid crystal display, and the precipitations are increased on only a region where the radiation heat was intensively applied, and are thus often recognized as irregularity on the surface. In addition, the required display brightness of liquid crystal displays in recent years is higher than 200 candelas, and the liquid crystal display on the side of the light source is exposed to light having an intensity of about 10,000 candelas. Depending on the temperature in the use environment, heat at about 40 to 60° C. is applied continuously to the liquid crystal display on the side of a light source. Accordingly, the precipitation of the ultraviolet absorbent was recognized not only in a heating reliability test but also in a continuous lighting test of the liquid crystal film mounted in a liquid crystal display. For example, if a UV-ray cured polymer obtained from a cholesteric liquid crystal composition blended with an ultraviolet absorbent is placed in an environment of 80° C.×500 hours or 60° C., 90% RH×500 hours, then cloudiness, an increase in haze, and precipitation of powder on the surface were significantly observed.